Forest declines are difficult to diagnose and can take several years to confirm, even for professionals and researchers. Three pieces are needed for positive confirmation: the factors that made the forest susceptible to decline, such as high tree density and soil compaction; the instigating factor, such as severe drought; and the insect or disease that ultimately kills the trees, such as twolined chestnut borer.
The first symptom of decline often is subtle dieback at the edge of the canopy. Other initial symptoms could be stunted leaves at the ends of some branches. These subtle signs are easily missed.
The year after this limited dieback, a large branch or branches die, usually towards the top of the crown. If drought is severe and the tree is highly susceptible to decline, the top two-thirds of the crown may die, or in extreme cases, the entire tree dies over one summer. This more drastic dieback often is first seen from mid-July to mid-August.
If the tree is still alive the second year of decline symptom development, more of its canopy may die in each subsequent year. Alternatively, dieback may stop, and the tree may persist with a dead top for many years and possibly recover. Many factors determine if decline symptoms stop or eventually result in death. Some of these factors are the level of stress the tree experienced before decline started; tree age; and the severity, duration, and number of different things that caused the decline.
A key symptom of decline in oaks, especially those in the red oak group, is that dead leaves remain on the branches for months. The persistence of dead leaves that hang on for months in the later summer and fall distinguishes Armillaria and twolined chestnut borer attack from oak wilt infection on oaks in the red oak group
Symptom progression of declining white and bur oaks can be nearly identical to oak wilt. In many cases, experienced tree pathologists rely on a lab confirmation to prove oak wilt is the cause. Signs of twolined chestnut borer and Armillaria don't rule out the presence of oak wilt, or the possibility that it is the main cause of mortality in the tree.
We highly recommend that landowners get a lab confirmation of oak wilt before applying oak wilt treatments.
This factsheet details how to distinguish between twolined chestnut borer and oak wilt.
Larvae of twolined chestnut borer can be found in dead and dying branches and trunks, where they make winding feeding tunnels. Adult beetles chew out of the bark, leaving D-shaped exit holes. Images of the insect's life stages can be seen at the University of Minnesota Extension bronze birch borer and twolined chestnut borer.
Armillaria, when actively causing disease, forms white fungal sheets (called mycelial fans) in the cambial zone of roots and the lower trunk of trees. Images of Armillaria life stages can be seen at the University of Minnesota Extension’s Armillaria root rot.
Oaks that die from oak wilt and Armillaria root disease are susceptible to secondary attack by twolined chestnut borer; the presence of exit holes and galleries does not necessarily mean that twolined chestnut borer killed the tree.
